Acommondiseasethatillustratesthedermatomalorganizationofthedorsalrootsisshingles. Shinglesistheresultofreactivationoftheherpeszostervirus,whichtypicallycauseschickenpoxduringtheinitialinfection.Duringtheinitialinfectionthevirusinfectsdorsalroot(andcranialnerve)ganglioncells,whereitcanremainlatentforyearstodecades.Whenthevirusreactivates,thecellsofthatparticulardorsalrootganglionbecomeinfected,andthevirustravelsalongtheperipheralaxonbranchesandgivesrisetoapainfuloritchyrashthatisconfinedtoonesideofthebody(endsatthemidline)inadermatomal,belt-likedistributionortothedistributionofacranialnerve.
Within the dorsal roots, fibers are not randomly distributed. Rather the large myelinated primary afferent fibers assume a medial position in the dorsal root, whereas the small myelinated and unmyelinated fibers are more lateral. The large medially placed afferent fibers enter the dorsal column, where they bifurcate into rostrally and caudally directed branches. These branches give off collaterals that terminate in several neighboring segments. The rostral branch also ascends to the medulla as part of the dorsal column–medial lemniscus pathway. The axonal branches that terminate locally in the spinal cord gray matter transmit sensory information to neurons in the dorsal horn and also provide the afferent limb of reflex pathways (see
Chapter 9).
Innervation of the Face
The arrangement of primary afferent fibers that supply the face is comparable to that of fibers that supply the body and is provided for primarily by fibers of the trigeminal nerve. Peripheral processes of neurons in the trigeminal ganglion (also called the gasserian or semilunar ganglion) pass through the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve to innervate dermatome-like regions of the face. These fibers carry both tactile information and pain and temperature information. The trigeminal nerve •Fig. 7.4 A,Dermatomesrepresentedonadrawingofapersonassumingaquadrupedalposition.NotenerveC1generallyhaslittleornosensorycomponent,andtheunlabeledportionoftheheadandthefaceareinnervatedbysensoryfibersofthecranialnerves,primarilythetrigeminalnerve.B,SagittalviewofthespinalcordshowingtheoriginofnervescorrespondingtoeachofthedermatomesshowninA.
Thetrigeminalnuclearcomplexconsistsoffourmaindivisions,threeofwhicharesensory.Thethreesensorydivisions(fromrostraltocaudal)arethemesencephalic, chief (ormain)sensory, anddescending (orspinal)trigeminal nuclei. Thelattertwoaretypicalsensorynucleiinthatthecellbodiescontainedinthemaresecond-orderneurons.Themesencephalicnucleusactuallycontainsfirst-orderneuronsandthusisanalogoustoadorsalrootganglion.Thefourthdivisionofthetrigeminalcomplexisthemotornucleusofthetrigeminalnerve,whosemotorneuronsprojecttoskeletalmusclesoftheheadviathetrigeminalnerve(see
Fig.4.6D–E also innervates the teeth, the oral and nasal cavities, and the cranial dura mater.